I’m not convinced what you run into is a specific podman issue. It’s a resource issue and configuration issue likely. “vanila” podman with proper rootless containers will run as much workload as machine can handle from my experience. My company costomers seem to be running production workloads with it just fine.
Oh wait, by rootless container you really meant running podman rootless? still don’t see an issue though. What specifically are you doing? I mean, what’s the configuration and what’s the workload?
My opinion is that of the two Postres is more “adult”. So if you want to"just wing it" MariaDB would work, but if you’re serious Postgres is a better choice. However Postgres also requires better understanding of you setup etc. So it’s a ROI game - what’s more important to your project, how complex your DB is, what are the requirements for availability, transaction security etc. There is no “better” or “worse” there’s “feasible” and “prohibitive” 😉